Method of making an electrode having a coating of cobalt metatitanate thereon

ABSTRACT

A coherent mixture of crystalline cobalt metatitanate with a valve metal oxide-platinum metal oxide solid solution is provided. Typical use of the mixture is as an adherent, electrically-conductive, electrocatalytically-active coating applied to an electrically-conductive substrate for use as an electrode.

United States Patent 1191 Kolb et a1.

[ 1 Dec.2,1975

1 1 METHOD OF MAKING AN ELECTRODE HAVING A COATING OF COBALTMETATITANATE THEREON [75] Inventors: James M. Kolb, Mentor; Kevin J.

OLeary, Cleveland Heights, both of Ohio [73] Assignee: ElectronorCorporation, Panama City, Panama [22] Filed: July 31, 1973 [21] Appl;No.: 384,210

Related U.S. Application Data [60] Division of Ser. No. 222,995, Feb. 2,1972, Pat. No. 3,778,363, which is a continuation-in-part of Scr. No.104,743, Jan. 7, 1971, abandoned.

[52] U.S. Cl. 427/126; 204/290 F; 427/226; 427/374; 427/372 [51] Int.Cl. B44D 1/18 [58] Field of Search 117/201, 215, 221; 204/290 F;427/126, 226, 372, 374

[56] References Cited UNITED STATES PATENTS 3,399,966 9/1968 Suzuki eta1. 117/221 FOREIGN PATENTS OR APPLICATIONS 1,147,442 4/1969 UnitedKingdom 204/290 F Primary Examiner-Cameron K. Weiffenbach Attorney,Agent, or Firm-Hammond & Littell [57] ABSTRACT A coherent mixture ofcrystalline cobalt metatitanate with a valve metal oxide-platinum metaloxide solid solution is provided. Typical use of the mixture is as anadherent, electrically-conductive, electrocatalyti- Cally-active coatingapplied to an electricallyconductive substrate for use as an electrode.

8 Claims, N0 Drawings METHOD OF MAKING AN ELECTRODE HAVING A COATING OFCOBALT METATITANATE THEREON REFERENCE TO A COPENDING APPLICATION This isa divisional application of our copending application Ser. No. 222,995filed Feb. 2, 1972, now US. Pat. No. 3,778,363 which in turn is acontinuation-inpart of our copending Ser. No. 104,743, filed Jan. 7.1971, now abandoned.

BACKGROUND OF THE INVENTION In the search for a satisfactorydimensionally stable electrode for use in a number of commercialelectrolytic processes an electrode has recently been developed whichhas met with considerable commercial success. This electrode consists ingeneral of a valve metal substrate bearing on its surface a valve metaloxideplatinum metal oxide solid solution-type coating. In such coatings,atoms of platinum metal are randomly substituted for atoms of valvemetal in the characteristic rutile valve metal oxide crystal lattice.Such electrodes exhibit remarkably low potentials when employed in avariety of electrolytic processes, e.g., as anodes for the production ofchlorine by electrolysis of sodium chloride solutions. A furtheradvantage of such electrodes is that they exhibit a relatively lowwearrate, that is, only a small amount of plantinum metal is consumedper ton of product manufactured.

Such electrodes are not entirely without disadvantage, however, in thatowing to the amount of platinum metal required to be incorporated in thecoating, the electrodes are expensive to fabricate. In addition, whilethe wear-rate is low, generally, for example, on the order of 0.10-0.15gram of platinum metal per ton of chlorine produced, considering thetonnages involved a significant amount of platinum metal isirretrievably lost. Further, once the platinum metal content of thesesolid solutions coatings is substantially depleted, the electrodesbecome inactive. Hence, the cell must be disassembled for removal andreplacement of the electrodes. For these reasons, the search for acoating material exhibiting all the advantages of the solid solutiontypecoating, with the further advantage of a reduced wear-rate, continues.

STATEMENT OF THE INVENTION Therefore it is an object of the presentinvention to reduce the amount of platinum metal required in a valvemetal oxide-platinum metal oxide solid solution without an attendantincrease in potential.

It is a further object of the present invention to provide a materialsuitable for use as an electrode coating and having a reduced platinummetal wear-rate.

These and further objects of the present invention will become apparentto those skilled in the art from the specification and claims whichfollow.

A composition has now been found which is especially useful forapplication as an electrode coating, which composition consistsessentially of a coherent mixture of crystalline cobalt metatitanatewith a valve metal oxide-platinum metal oxide solid solution. Whenapplied to a supporting substrate, especially an electrically-conductivesupporting substrate and particularly a valve metal substrate, anelectrode is obtained which exhibits a low potential and which hasextremely low platinum metal wear-rates.

2 It has further been found that such an electrode is particularlyeffective if applied in a manner which allows rapid initial heating ofthe coating during formation and subsequent post-treatment at anelevated tem- 5 perature. In this manner, the physical form of thecobalt metatitanate is apparently optimized, resulting in a particularlydurable coating.

The invention finds particular advantage when the coherent mixture ofcrystalline cobalt metatitanate with a valve metal oxide-platinum metaloxide solid solution is applied to a valve metal base, the resultingstructure being used as an anode for the production of chlorine byelectrolysis of brine. In this manner, a reduction in the amount ofplatinum metal required per square foot of anode surface is possible,without sacrifree in the potential at which chlorine is discharged atthe anode face. Surprisingly, the anode has a significantly lowerplatinum metal wear-rate per ton of chlorine produced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A sample electrode has beenprepared as is described more fully hereinbelow by application to aclean titanium substrate of a solution containing salts of titanium,ruthenium and cobalt. The electrode exhibits re markable adhesionbetween the coating and the substrate. Examination by X-ray diffractionindicates, in addition to the presence of the characteristic rutile Ti-O solid solution crystal structure, that there is also presentcrystalline cobalt metatitanate as a separate phase dispersed throughoutthe solid solution. It is apparent from the results obtained that thecobalt metatitanate in some manner stabilizes the solid solution, thuspreventing normal wear to a great extent. To qualify as crystalline forthe purpose of this invention the metatitanate, when subjected to X-raydiffraction analysis, must exhibit peaks characteristic of cobaltmetatitanate. It is not necessary that the crystals be highly ordered,although this is preferred, only sufficiently so that discemable peaksare evidenced. While cobalt metatitanate having at least some degree ofcrystallinity is required and constitutes the invention in admixturewith the solid solution, cobalt oxides and/or amorphous cobalt titanatemay be present without substantial detriment. As is explained more fullyhereinbelow, the extent of cobalt titanate crystallinity appears todepend upon the various heat treatments to which the electrode issubjected during preparation.

The solid solutions contemplated by the present invention have beendescribed as valve metal oxideplatinum metal oxide solid solutions. Byvalve metal it is intended to refer to titanium, tantalum, zirconium andniobium, while by platinum metal the reference is to platinum,palladium, ruthenium, iridium, rhodium and osmium.

The quantities of the various ingredients of the composition of thepresent invention are conveniently expressed in terms of the mole ratiosof the various metals present. Thus the ratio of valve metal to platinummetal plus cobalt should be within the range of 5: 1-1 :5 while theratio of platinum metal to cobalt should be within the range of 4:1-2:3. A preferred range, especially when dealing with titanium, rutheniumand cobalt is, respectively, 10:2:3. Expressed another way, thecrystalline cobalt metatitanate will constitute up to 35%, preferably10-35%, still more preferably 25-35%, by weight, of the total solidsolution plus crystalline cobalt 3 metatitanate.

Generally the use of the compositions of the present invention will beas a coating applied to a conductive substrate. However, the materialitself, absent a substrate, will find some application. Techniques forpreparation are in many respects standard to the preparation of solidsolutions, with the additional inclusion of a cobalt compound. Forexample sealed tube or flame spray techniques may be applied. Thematerial thus obtained has a high degree of non-selective catalyticactivity thus suggesting its use for the catalytic destruction oforganic compounds, such as in exhaust emission. Alternately, thematerial may be impregnated into porous graphite for use as a fuel cellelectrode or dispersed in an organic binder, such as a fluorocarbonbinder, to prepare a porous electrode, again useful in fuel cells. Otherapplications will suggest themselves to those skilled in the art.

Generally, however, the composition finds applica tion as a coating on asupporting substrate. Since the coating will not generally besufficiently massive to conduct current from the primary souce, anelectrically-conductive substrate will be employed. Additionally, thesubstrate will be one which, upon the possible mechanical failure of theapplied coating, will be inert to the environment in which it isemployed. Preferably, at least when use is to be as an anode for theproduction of chlorine, the substrate will be a valve metal, typicallytitanium. The physical configuration of the supporting substrate isindependent of the present invention and depends primarily upon the celldesign in which the electrode is to be employed. It is also quitepossible to apply the coating to a substrate to which there haspreviously been applied a protective or barrier layer to achieve adesired chemical or physical effect, for example, a manganese dioxidelayer serving to prevent the diffusion of oxygen through the coating tothe underlying substrate. Alternately, protective ceramic layers may beapplied over the coating, as is known in the art.

While the invention is not so limited, the preferred method of applyingthe coating composition of the present inve ntion to a supportingsubstrate, particularly a valve metal substrate, involves thethermochemical deposition of the coating in a number of successivelayers. Generally in such a technique, salts of the various componentmetals are dissolved, for example in alcohol, and applied, as bypainting onto the substrate. The solution-coated substrate is thenheated to an elevated temperature in the presence of oxygen tothermochemically convert the salts into the desired chemical andphysical form. Successive layers, e.g., 4-l 2, are applied in thismanner until the desired coating thickness is obtained.

While the foregoing general technique has been used with success in thepreparation of the usual unmodified solid solution-type coatings, it hasbeen found that certain alterations in the procedure are required ifcrystalline cobalt metatitanate is to be obtained. Ordinarily thesubstrate with the applied solution is introduced relatively graduallyinto an oven and brought to the temperature required for thermochemicalconversion. It has been found that to prepare a crystalline cobaltmetatitanate-modified coating, however, it is necessary to raise thetemperature of the substrate rapidly to within the desired final range.In this manner, a larger proportion of cobalt metatitanate in thedesired crystalline structure, as opposed to oxides or amorphoustitanates, is formed. In addition, it has been found that, afterapplication of the last solution coating and conversion to the desiredchemical form, the coated substrate is preferably subjected to apost-bake treatment at an elevated temperature which treatment furtherorders the crystals. It has been noted that electrodes subjected to thispost-bake exhibit improved wear-rates. It is thought to be surprisingthat crystalline cobalt metatitanate results from this procedure sinceknown means for production of same generally require temperatures withinthe range of 800-l000C.

Thus, for example, a preferred method of preparing an electrodecomprises:

a. applying a solution of salts of titanium, ruthenium and cobalt to anelectrically-conductive substrate;

b. rapidly inserting the substrate with applied solution into an oven ata temperature within the range of 425475C and in the presence of oxygen;

c. maintaining the substrate at the oven temperature for a period oftime sufficient to thermochemically convert the salts to the desiredchemical state;

d. cooling the substrate;

e. applying any desired number of coatings of said solution in the samemanner and,

f. after thermochemical conversion of the last solution application,heating the substrate to a temperature of 525575C for from 5 to 10minutes.

In order that those skilled in the art may more readily understand thepresent invention, the following specific examples are afforded. Inthese examples, wear-rates are determined by applying the coatings to asix inch square expanded titanium substrate and employing same as theanode in a horizontal mercury cell. The anode-cathode gap is establishedat 0.15 inch, the current density at 6 amperes per square inch, thetemperature of the brine (290 grams per liter aqueous sodium chloride)at 160F., the brine flow rate at 425 milliliters per minute and themercury flow rate at 450 milliliters per minute. Prior to insertion inthe cell, the anode is repeatedly washed, dried and weighed untilweights agreeing within 0.1 milligram are obtained. Operation is thencommenced and the anodes are removed when desired, usually everyoperating hours, and reweighed to the same criteria, the differencerepresenting the wear of the anode.

EXAMPLE 1 Coating solutions are prepared by dissolving tetrabutylorthotitanate, RuCl .2.5 H 0 and CoCl .6I-I O in normal butyl alcoholcontaining about 6 percent by volume of 36 percent I-ICl. The amounts ofthe titanium, ruthenium and cobalt salts used are those sufficient togive the mole ratios indicated in Table 1, i.e., for Sample E, 317, 63and 47 grams per liter, respectively, of the titanium, cobalt andruthenium salts are used. Six coats of each solution are applied toclean titanium mesh substrates with heating after each application inair to a temperature of 450C. The temperature of the substrate isbrought rapidly (one minute or less) to temperature, which is maintainedfor seven minutes. After application of the last coating, samples B-Fare post-baked for 7 minutes at 550C. Sample A is not so treated sinceit is known that heating at elevated temperatures has an adverse effectupon the potential of standard solid solution-type coatings. On theother hand, in Sample G thermochemical decomposition is effected at 300Cand the post-bake temperature is 600C. The values given for chlorinepotential are obtron.

TABLE I Moles Moles Moles Sample Ti Ru Co C0 C1 Substitution Potential F2 0.2 0.8 80 l .49 G 2 0 l 100 l 5 EXAMPLE 2 This example illustratesthe importance of temperature control in the thermochemical depositionmethod of the present invention. Anodes are prepared as in Sample E ofExample 1 with the exception that (a) Sample El is merely baked betweencoats for seven minutes at 450C; (b) Sample E2 is baked for sevenminutes between coats at 450C and post-baked for seven minutes at 550Cand, (c) Sample E3 is baked for seven minutes between coats is 550C.X-ray analysis shows the following coating characteristics El) rutilesolid solution (TiO --RuO no crystalline cobalt compounds; E2) rutilesolid solution, crystalline cobalt metatitanate, E3) rutile solidsolution, crystalline cobalt metatitanate. The chlorine potential ofsample E3, measured at 6 a.s.i., is 0.06 volt higher than sample E2.Furthermore, a solubility test conducted by boiling the samples for 10minutes in 10 volume percent HCl indicates that 0.40 milligram ofruthenium and 4.5 milligrams of cobalt are leached from sample Elwhereas only 0.13 and 3.2 milligrams, respectively, are leached from E2and E3.

EXAMPLE 3 In this example, certain of the samples from Examples l and 2are subjected to the wear-rate test previously described with theresults shown in Table 11.

TABLE 11 TABLE ll-c0ntinued Cumulative Wt. Loss (mg) at V (hrs.) 6a.s.i. After 500 Sample Co 200 300 Hrs.

D 50 1.8 4.3 5.4 1.42 E2 60 1.9 3.0 4.5 1.48 E1* 60 8.8 13.4 17 1.50

*No post-bake Two conclusions are reached. The crystalline cobaltmetatitanate-modified solid solution coatings are superior to theunmodified coatings of the prior art insofar as wear-rate is concerned,often without a sacrifice in voltage. Further, while its voltage isstill favorable, a cobalt-modified coating, absent the post-baketreatment and hence non-crystalline, exhibits a high wearrate (E1).

Although the invention has been described with reference to certainpreferred embodiments thereof, it is not to be so limited since changesand alterations may be made therein which are still within the full andintended scope of the appended claims.

We claim:

1. A method of preparing an electrode which comprises:

a. applying a solution of salts of titanium, ruthenium and cobalt to anelectrically-conductive substrate;

b. rapidly heating the substrate with applied solution to a temperaturewithin the range of 425475C and in the presence of oxygen;

c. maintaining the substrate at the said temperature for a period oftime sufficient to thermochemically convert the salts to a coherentmixture of crystalline cobalt metatitanate with a valve metaloxide-precious metal oxide solid solution;

d. cooling the substrate;

e. applying any desired number of coatings of said solution in themanner of steps (a) to (d) and,

f. after thermochemical conversion of the last solution application,heating the substrate to a temperature of 525-575C for from 5 to 10minutes,

whereby there is obtained an electrode bearing on at least a portion ofits surface an electrically-conductive, electro-catalytically-activecoating consisting essentially of a coherent mixture of crystallinecobalt metatitanate and a titanium dioxide-ruthenium dioxide solidsolution.

2. A method as in claim 1 wherein the titanium, ruthenium and cobaltsalts are, respectively, butyl titanate, ruthenium chloride and cobaltchloride.

3. A method as in claim 1 wherein the titanium, ruthenium and cobaltsalts are used in amounts sufficient to provide a mole ratio in solutionof titanium; ruthenium plus cobalt within the range of 5: 1-l :5 and aratio of ruthenium; cobalt within the range of 421-223.

4. A method as in claim 1 wherein the temperature in Step (b) is about450C.

5. A method as in claim 1 wherein the temperature in Step (f) is about550C.

6. A method as in claim 1 wherein according to Step (e) from 4-12 coatsare applied.

7. A method as in claim 1 wherein the substrate is a valve metalsubstrate.

8. A method as in claim 1 wherein the substrate is ti-

1. A METHOD OF PREPARING AN ELECTRODE WHICH COMPRISES: A. APPLYING ASOLUTION OF SALTS OF TITANIUM, RUTHENIUM AND COBALT TO ANELECTRICALLY-CONDUCTIVE SUBSTRATE; B. RAPIDLY HEATING THE SUBSTRATE WITHAPPLIED SOLUTION TO A TEMPERATURE WITHIN THE RANGE OF 425*-475*C AND INTHE PRESENCE OF OXYGEN C. MAINTAINING THE SUBSTRATE AT THE SAIDTEMPERATURE FOR A PERIOD OF TIME SUFFICIENT TO THERMOCHEMICALLY CONVERTTHE SALTS TO A COHERENT MIXTURE OF CRYSTALLINE COBALT METATITANATE WITHA VALVE METAL OXIDE-PRECIOUS METAL OXIDE SOLID SOLUTION D. COOLING THESUBSTRATE; E. APPLYING ANY DESIRED NUMBER OF COATINGS OF SAID SOLUTIONIN THE MANNER OF STEPS (A) TO (D) AND, F. AFTER THERMOCHEMICALCONVERSION OF THE LAST SOLUTION APPLICATION, HEATING THE SUBSTRATE TO ATEMPERATURE OF 525*-575*C FOR FROM 5 TO 10 MINUTES, WHEREBY THERE ISOBTAINED AN ELECTRODE BEARING ON AT LEAST A PORTION OF ITS SURFACE ANELECTRICALLY-CONDUCTIVE, ELECTROCATALYTICALLY-ACTIVE COATING CONSISTINGESSENTIALLY OF A COHERENT MIXTURE OF CRYSTALLINE COBALT METATITANATE ANDA TITANIUM DIOXIDE-RUTHENIUM DIOXIDE SOLID SOLUTION.
 2. A method as inclaim 1 wherein the titanium, ruthenium and cobalt salts are,respectively, butyl titanate, ruthenium chloride and cobalt chloride. 3.A method as in claim 1 wherein the titanium, ruthenium and cobalt saltsare used in amounts sufficient to provide a mole ratio in solution oftitanium; ruthenium plus cobalt within the range of 5:1-1:5 and a ratioof ruthenium; cobalt within the range of 4:1-2:3.
 4. A method as inclaim 1 wherein the temperature in Step (b) is about 450*C.
 5. A methodas in claim 1 wherein the temperature in Step (f) is about 550*C.
 6. Amethod as in claim 1 wherein according to Step (e) from 4-12 coats areapplied.
 7. A method as in claim 1 wherein the substrate is a valvemetal substrate.
 8. A method as in claim 1 wherein the substrate istitanium.